The E. coli SOS system is an inducible system comprising some 20+ genes that are normally repressed but become induced when ongoing DNA replication is blocked by DNA lesions. It can also be expressed constitutively by genetic means in strains carrying a constitutively activated RecA protein. Part of the response is an error-prone activity that leads to greatly enhanced mutagenesis on both damaged (translesion synthesis) and undamaged DNA. Genetic experiments have revealed that this error-prone activity requires the action of the SOS-inducible UmuC, UmuD' and RecA proteins as well as DNA synthesis by DNA polymerase III. However, the precise nature of the mutagenic events is unknown. We have performed genetic experiments on the SOS mutator activity produced by the constitutively activated recA730 mutation in the absence of exogenously supplied DNA damage. These experiments suggest that the SOS mutator activity does not enhance the intrinsic error rate of replication, but instead amplifies normal polymerase errors through promotion of extension from mispaired 3' termini while preventing the proofreading or dissociation pathways. We propose that a similar mechanism operates when the polymerase encounters DNA lesions. The precise interaction of the RecA, UmuD and UmuC proteins with polymerase III is currently probed by searching for mutants in polymerase III subunits which specifically lead to either impaired (antimutator) or increased (mutator) of the SOS mutator effect.